Rock that is removed from a quarry is typically hauled to a rock crushing plant. Within the rock crushing plant, there are usually three stages of crushing: primary crushing, secondary crushing, and tertiary crushing. Some of the rock removed from the quarry is of a sufficient size to enter the secondary and tertiary crushing stages directly. The remainder of the rock removed from the quarry must pass trough a primary crushing stage so that it is reduced in size, typically to 8 inches in diameter or less.
All of the rock removed from a quarry or other location is dumped by a hauling vehicle into a hopper or bin. This includes the rock that is already of a sufficient minimum size to be introduced into the secondary and tertiary crushing stages directly. The rock is emptied out of the hopper by a feeder across screening bars that allow the sufficiently small sized rock to pass through to a conveyer that transfers the smaller diameter rock to the other crushing stages in the plant. The remainder of the rock is fed to a primary crusher, such as a Jaw Crusher or a Gyratory Crusher.
The throughput of the primary crusher therefore governs the amount of rock that can be introduced into the secondary and later stages of crushing. Accordingly, when the primary crushing stage is unable to deliver enough rock to the secondary and later stages of crushing within the rock crushing plant, the overall efficiency of the plant is greatly reduced. Conversely, when the feeder of the primary crusher is operated at a flow rate that produces a large volume of crushed rock being discharged to an output conveyer, the output conveyer or other output side equipment becomes overloaded so the feeder rate needs to be decreased. If the output conveyer reaches an overloaded condition, then the motor driving the conveyer trips off in response to an overload prevention circuit. This creates unwanted down time and the likelihood of material spillage.
Ordinarily, an operator is required to regulate the feed rate of rock being supplied to the primary crusher and to supervise the dumping of rock into the hopper that is delivered by the haul vehicles returning from the quarry. The operator relies upon his experience to vary the feed rate in order to maintain a constant supply of rock to the primary crusher and to keep the hopper full without overfilling it. The output of the primary crusher, therefore, is controlled by the operator. The operator has no way to determine whether the first stage crushing operation is at optimum efficiency other than to depend on his experience in operating the equipment. Further, the operator usually runs the primary crusher feeder at a rate that prevents an unwanted overload from ever occurring, and thus also prevents the throughput of the primary crusher from ever reaching an optimum throughput.
It has been known to automate secondary and tertiary crushing stages in a rock crushing plant by controlling the feed rate of a feeder delivering rock to the secondary or tertiary crushers being used. For example, U.S. Pat. No. 4,804,148 discloses that control systems are known wherein a programmable logic controller has been used to vary the feed rate to a secondary or tertiary crusher in accordance with signals received from a horsepower sensor and level sensor so that an optimum feed rate for the conditions being sensed can be determined and set for the crusher feeder. According to the known methods, however, a material contacting level sensor is required to extend within the bowl of the crusher in order to determine the level of rock within the crusher bowl. The level sensing probe that has been used is of a design that is able to withstand the harsh environment encountered in a rock crushing bowl of a secondary or tertiary crusher material contacting level sensor. It is not known, however, to sense the level of rock within a crushing cavity for a primary crusher, because the rock that enters a primary crusher is much larger than the rock entering a secondary or tertiary crusher thus creating an even harsher environment for the level sensor. Accordingly, no attempts have been made to automate a primary crusher, so an operator has always been used to regulate the feed rate delivered to a primary crusher and to signal the driver of a haul vehicle when to dump his load.